Baculum

Baculum, also called Os Penis, orOs Priapi, the penisbone of certain mammals. The baculum is one of several heterotropic skeletal elements—i.e., bones dissociated from the rest of the body skeleton. It is found in all insectivores (e.g., shrews, hedgehogs), bats, rodents, and carnivores and in all primates except humans. Such wide distribution suggests that it appeared early in mammalian evolution.

Raccoon baculum.

Mordicai

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...rigidity. Among mechanisms that reverse the erectile state are disgorgement of blood from the cavernous spaces, elasticity of the walls of the spaces, and action of a retractor muscle. A penis bone (baculum, os priapi) is present in various degrees of development in many mammals.

Many carnivores have a well-developed penis bone, or baculum. It appears that this structure plays a role in helping to increase the success of copulation and fertilization of eggs in species where numerous males mate with a single female. Cats have a vestigial baculum or none at all, but the baculum of the walrus can measure up to 54 cm (21 inches).